Ona a map of Scotland, published recently by the Scottish Executive, an entire swath of land appears to have disappeared and fallen mysteriously into the sea. Coincidentally, it happens to be the country's second most marginal constituency. A mere 50 votes are at stake in Galloway and Upper Nithsdale, the sprawling constituency in the south-west of Scotland that stretches from the former mining communities of Kirkconnell and Sanquhar in the north, through the hills, valleys and rolling farmland around the thriving market town of Castle Douglas, to the busy port of Stranraer in the west.

On the face of it, Labour has little to fight for here anyway. It is a clear two-horse race between the Tories and the SNP. At the last Holyrood election, Alex Fergusson, a farmer turned Tory MSP won the seat with a majority of only 99. He took 11,332 votes over the SNP's Alasdair Morgan, who got 11,233 votes.

But in many ways it is the disaffected Labour voters who will decide what happens in this constituency, as in the entire election. As such, it is a key target for the Nationalists.

Fergusson is acutely aware of this and has made the battle to save the Union, rather than simply holding on to his seat, the key issue in his fight with his opponent. In the last election, his campaign slogan was 'Keep Galloway SNP-free in 2003'. This time it is 'Keep independence at bay, vote Alex in May.' Interestingly, he admitted than on many issues, such as local transport, health and housing concerns, there is no 'clear blue water' between him and his SNP opponent. The one fundamental difference between them is the constitution.

'The constitutional question was not something I was planning to make a big deal of,' he said. 'But it has been raised so often with me. A lot of non-SNP voters are seriously concerned about the consequences of an SNP victory. Non-Nationalists are afraid that we are in danger of sleepwalking towards independence. I don't want that and I don't think most people in this constituency want it.'

Even Fergusson conceded that it's too close to call, admitting he wouldn't be surprised if he won or lost. However, he said: 'I keep reading of this massive SNP lead all across the country, but I don't find it on the streets. Genuinely. You can't be certain of anything when you have a majority of 99, but I'm increasingly confident that we can hold onto this seat.'

Not surprisingly, Morgan, who was the MP for the area after defeating then Tory Scottish Secretary Ian Lang in 1997 and who won the seat in the first Holyrood election in 1999, does not agree. He accused his opponent of 'gilding the lily'. 'In his campaign leaflets and posters, he's not emphasising his unionism. He's stressing that he's a local. In fact, on posters, his name is in huge writing, but you would need a magnifying glass to read the tiny letters at the bottom saying Conservative party. This is simply because the Tories are still seen as an anti-Scottish party.'

As well as seeking out the 709 Scottish Socialist Party votes from last time, Morgan also believes the Nationalists will take votes from disgruntled Labour voters as well as Tories. 'People desperately want a change. The south-west cannot and will not progress until Scotland does and people are realising that there is only one opposition to Labour. I have spoken to voters who have previously voted Tory but who realise now it is a wasted vote. There is only one option to get Labour out and that is the SNP.'

Ian Livingston, a co-owner of an antique restorers and furniture makers in Castle Douglas, said in the past he had voted SNP but was undecided this time. 'I'm not convinced anything tremendously positive would come out of it,' he said. 'Instead I think it would just result in more political in-fighting and there would be a huge cost involved. As a businessman you want stability.'

But Roddy MacLeod, 31, who moved to Castle Douglas from Edinburgh seven months ago said 'it's time' for a change, mimicking the SNP's campaign slogan. 'I will definitely use my vote tactically,' he said.

If the Scottish Executive thinks this area has ceased to exist, angry locals, who have long felt neglected by their political leaders in Edinburgh, feel differently. A spokesman said the map was merely a guide and not a representation of Scotland. 'This is no way meant as a slight against Dumfries and Galloway,' he added. 'We definitely want to hear people's views.' On 3 May they will.